How can I cast shadow on background with nodes
Once you have your subject cut out, you can use its alpha channel to make a shadow layer with the compositor:




After the color keying is taken care of, use the Separate RGBA node to get the alpha channel.
Run it through a translate node so you can move the shadow.
The alpha channel will need to be inverted with an invert node.
Multiply the shadow with the background and use the multiply factor to control the opacity of the shadow.
Run the shadow-background and color key results through an alpha over node to put the color keyed result over the background-shadow.